by Chris Taylor If you had to name a handful of major events that reshaped the financial world over the past few decades, one of them would have to be the rise of index, or passive, investing. As opposed to active stock picking — or selecting individual companies you think are poised for growth — stock market index funds simply track the entire groups of securities. So if you buy shares in an S&P 500 index fund, for instance, you will own a tiny slice of all 500 companies on that list.